Individually routable subcarriers

ABSTRACT

Consistent with an aspect of the present disclosure, electrical signals or digital subcarriers are generated in a DSP based on independent input data streams. Drive signals are generated based on the digital subcarriers, and such drive signals are applied to an optical modulator, including, for example, a Mach-Zehnder modulator. The optical modulator modulates light output from a laser based on the drive signals to supply optical subcarriers corresponding to the digital subcarriers. These optical subcarriers may be received by optical receivers provided at different locations in an optical communications network, where the optical subcarrier may be processed, and the input data stream associated with such optical subcarrier is output. Accordingly, instead of providing multiple lasers and modulators, for example, data is carried by individual subcarriers output from an optical source including one laser and modulator. Thus, a cost associated with the network may be reduced. Moreover, each of the subcarriers may be detected by a corresponding one of a plurality of receivers, each of which being provided in a different location in the optical communication network. Thus, receivers need not be co-located, such that the network has improved flexibility.

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Patent Application No. 62/569,847, filed on Oct. 9, 2017,the entire content of which is incorporated by reference herein in itsentirety.

Optical communication systems are known in which multiple opticalsignals, each having a corresponding wavelength, and each beingmodulated to carry a different data stream, are multiplexed onto anoptical fiber. In such systems, a laser and a modulator may be used togenerate each optical signal. Accordingly, in order to increase thecapacity of such systems, additional lasers, modulators and associatedcircuitry are employed. The cost associated with such systems maytherefore increase, as capacity is increased. Accordingly, there is aneed for a more cost-effective network requiring fewer components, suchas those described above.

SUMMARY

Consistent with an aspect of the present disclosure, an apparatus isprovided that comprises a digital signal processor that receives aplurality of independent data streams, the digital signal processorsupplying outputs based on the plurality of independent data streams. Inaddition, a laser is provided, as well as a modulator that outputs amodulated optical signal based on the plurality of outputs. The opticalsignal including a plurality of Nyquist subcarriers based on the outputsof the digital signal processor, wherein a first one of the plurality ofNyquist subcarriers carries data indicative of a first one of theplurality of independent data streams, and a second one of the pluralityof Nyquist subcarriers carries data indicative of a second one of theplurality of independent data streams.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one (several) embodiment(s) andtogether with the description, serve to explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a mesh network consistent with anaspect of the present disclosure;

FIG. 2 illustrates each node shown in FIG. 1 in greater detail;

FIG. 3a illustrates a block diagram of a transceiver consistent with anaspect of the present disclosure;

FIG. 3b illustrates an example of a transmitter portion of thetransceiver shown in FIG. 3 a;

FIG. 4 illustrates an example of a digital signal processor (DSP)consistent with a further aspect of the present disclosure;

FIG. 5 illustrates a plurality of Nyquist subcarriers consistent with anaspect of the present disclosure;

FIG. 6 shows an example of a receiver consistent with an aspect of thepresent disclosure; and

FIG. 7 illustrates another example of a receiver consistent with afurther aspect of the present disclosure; and

DESCRIPTION OF THE EMBODIMENTS

Consistent with an aspect of the present disclosure, electrical signalsor digital subcarriers are generated in a DSP based on independent inputdata streams. Drive signals are generated based on the digitalsubcarriers, and such drive signals are applied to an optical modulator,including, for example, a Mach-Zehnder modulator. The optical modulatormodulates light output from a laser based on the drive signals to supplyoptical subcarriers corresponding to the digital subcarriers. Theseoptical subcarriers may be received by optical receivers provided atdifferent locations in an optical communications network, where theoptical subcarrier may be processed, and the input data streamassociated with such optical subcarrier is output. Accordingly, insteadof providing multiple lasers and modulators, for example, data iscarried by individual subcarriers output from an optical sourceincluding one laser and modulator. Thus, a cost associated with thenetwork may be reduced. Moreover, each of the subcarriers may bedetected by a corresponding one of a plurality of receivers, each ofwhich being provided in a different location in the opticalcommunication network. Thus, receivers need not be co-located, such thatthe network has improved flexibility.

Reference will now be made in detail to the present embodiment(s)(exemplary embodiments) of the present disclosure, an example(s) ofwhich is (are) illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

FIG. 1 illustrates a block diagram of a mesh network 100 including nodes10, 20, 30, and 40 consistent with an aspect of the present disclosure.As discussed in greater detail below, optical transmitters and receiversfurther consistent with the present disclosure may be provided in eachnode to reduce network cost and improved network flexibility. As shownin FIG. 1, each of nodes 10, 20, 30, and 40 may communicate with anotherone of these nodes.

FIG. 2 shows each of nodes 10, 20, 30, and 40 in greater detail. In oneexample, each node includes a respective transceiver, such that node 10includes transceiver 11, node 20 includes transceiver 21, node 30includes transceiver 31, and node 40 includes transceiver 41.Transceiver 11 may include an output that supplies modulated opticalsignals to a splitter 12, which, in turn, has a plurality of outputs orports 12-1, 12-2, and 12-3. Each of these ports may provide a powersplit portion of the input modulated optical signal to the splitter 12.As discussed in greater detail below, the modulated optical signal mayinclude a plurality of subcarriers. Such subcarriers may be Nyquistsubcarriers.

Node 10 may also include an optical combiner 14 that receives powersplit portions of modulated optical signals from other nodes 20, 30, and40 (described below) at inputs or ports 14-1, 14-2, and 14-3 andsupplies the combined portions as an input to transceiver 11.

Transceiver 21 of node 20 may include an output that supplies modulatedoptical signals to a splitter 22, which, in turn, has a plurality ofoutputs or ports 22-1, 22-2, and 22-3. Each of these ports may provide apower split portion of the input modulated optical signal to splitter22.

Node 20 may also include an optical combiner 24 that receives powersplit portions of modulated optical signals from other nodes 10, 30, and40 (described below) at inputs or ports 24-1, 24-2, and 24-3 andsupplies the combined portions as an input to transceiver 21.

Transceiver 31 of node 30 may include an output that supplies modulatedoptical signals to a splitter 32, which, in turn, has a plurality ofoutputs or ports 32-1, 32-2, and 32-3. Each of these ports may provide apower split portion of the input modulated optical signal to splitter32.

Node 30 may also include an optical combiner 34 that receives powersplit portions of modulated optical signals from other nodes 10, 20, and40 (described below) at inputs or ports 34-1, 34-2, and 34-3 andsupplies the combined portions as an input to transceiver 31.

Transceiver 41 of node 40 may include an output that supplies modulatedoptical signals to a splitter 42, which, in turn, has a plurality ofoutputs or ports 42-1, 42-2, and 42-3. Each of these ports may provide apower split portion of the input modulated optical signal to splitter42.

Node 40 may also include an optical combiner 44 that receives powersplit portions of modulated optical signals from other nodes 10, 20, and30 (described below) at inputs or ports 44-1, 44-2, and 44-3 andsupplies the combined portions as an input to transceiver 41.

Table 1 below lists the outputs of each splitter and correspondingcombiner inputs optically coupled to such splitter outputs for meshnetwork 100.

TABLE 1 Splitter Output Combiner Input 12-1 24-1 12-2 34-1 12-3 44-122-1 14-1 22-2 34-2 22-3 44-2 32-1 14-2 32-1 24-2 32-3 44-3 42-1 14-342-2 34-3 42-3 24-3

Accordingly, as shown in Table 1, as well as in FIG. 2, a power splitportion of the modulated optical signal (including a power split portionof each optical subcarrier included therein) is output from each ofports 12-1, 12-2, and 12-3 of splitter 12 (Node 1) to a correspondingone of input ports 24-1 (node 20), 34-1 (node 30), and 44-1 (node 40),respectively. In another example, a power split portion of the modulatedoptical signal (including a power split portion of each opticalsubcarrier included therein) is output from each of ports 32-1, 32-2,and 32-3 of splitter 32 (Node 1) to a corresponding one of input ports14-2 (node 10), 24-2 (node 20), and 44-3 (node 40), respectively.Similar connections are also made between the splitters and combiners ofnodes 20 and 40.

FIG. 3a shows an example of transceiver 11. It is understood thattransceivers 21, 31, and 41 may have the same or similar structure astransceiver 11. Transceiver 11 may include an optical transmittercircuit 301 that supplies modulated optical signals to splitter 12.Transceiver 11 may also include a receiver circuit 302 that receives theoptical output from combiner 302, for example. The receiver andtransmitter circuits may be housed together on the same card and/orhousing or may be provided in separate housings.

FIG. 3b shows transmitter circuit 301 in greater detail. Opticaltransmitter 301 may include a TX DSP 310, DACs 320-1 a,1 b and 320-2 a,b(referred to generally as DACs 320 and individually as DAC 320), a laser330, modulators 340-1 and 340-2 (referred to generally as modulators 340and individually as modulator 340), and splitter 350. TX DSP 310, DACs320, laser 330, and modulators 340.

Splitter 350 may include an optical splitter that receives continuouswave (CW) light, for example, from laser 330 and splits such light intotwo branches or portions: one for a first polarization (e.g., transverseelectric, TE) and one for the second polarization (e.g., transversemagnetic, TM). In some implementations, the two light portions may haveapproximately equal power. Splitter 350 may output one light portion tomodulator 340-1 and another light portion to modulator 340-2.

Modulator 340-1 may be used to modulate signals of the firstpolarization. Modulator 340-2 may be used to modulate signals for thesecond polarization. It is noted, however, that generally the lightoutput form laser 330 has one polarization, e.g., the firstpolarization, such that both modulators 340 provide modulated opticalsignal having the same polarization. Accordingly, a polarization rotatormay be provided at the input or the output of one of modulators 340, sothat the polarization of one of the modulated optical signals isrotated. A polarization beam combiner may also be provided to multiplexthe polarization rotated and unrotated modulated optical signals.

In some implementations, two DACs 320 (320) may be associated with eachpolarization. In these implementations, two DACs 320-1 a,b may supplyvoltage signals to modulator 340-1, and two DACs 320-2 a,b may supplyvoltage signals to modulator 340-2. The outputs of modulators 340 may becombined back together using combiners (e.g., optical multiplexer 216)and polarization multiplexing, as noted above.

While FIG. 3b shows optical transmitter 301 as including a particularquantity and arrangement of components, in some implementations, opticaltransmitter 212 may include additional components, fewer components,different components, or differently arranged components. The quantityof DACs 320, lasers 330, and/or modulators 340 may be selected toimplement an optical transmitter 301 that is capable of generatingpolarization diverse signals for transmission on an optical fiber, suchas a link between a pair of nodes in network 100 (see FIG. 1). In someinstances, one of the components illustrated in FIG. 3b may perform afunction described herein as being performed by another one of thecomponents illustrated in FIG. 3 b.

FIG. 4 shows an example of a DSP 310 in greater detail. As shown in FIG.5, TX DSP 310 may include an FEC encoders 405-1 to 405-4, a de-muxcomponent 410, an input bits component 420, a bits to symbol component430, an overlap and save buffer 440, a fast Fourier transform functions(FFT) component 450, a replicator component 460, a pulse shape filter470, a mux component 480, an inverse FFT (IFFT) component 490, and atake last 1024 component 495.

Each of FEC encoders 405-1 to 405-4 may receive a plurality ofindependent input data streams of bits (Client Data 1-4) from arespective one of a plurality of data sources and perform errorcorrection coding on a corresponding one of the input data streams, suchas through the addition of parity bits. FEC encoders 405-1 to 405-4 maybe designed to generate timing skew between the subcarriers to correctfor skew induced by link between nodes 10, 20, 30, and 40 in network100. De-mux component 410 may provide each group of bits to acorresponding input bits component 420. Input bits component 420 mayprocess 128*X bits at a time, where X is an integer. Fordual-polarization Quadrature Phase Shift Keying (QPSK), X is four. Forhigher modulation formats, X may be more than four. For example, for an8-quadrature amplitude modulation (QAM) format, X may be eight and for a16 QAM modulation format, X may be 16. Accordingly, for such 8 QAMmodulation, eight FEC encoders may be provided, each of which may encodea respective one of eight independent input data streams for acorresponding one of eight subcarriers. Likewise, for 16 QAM modulation,sixteen FEC encoders may be provided, each of which may encode arespective one of sixteen independent input data streams for acorresponding one of sixteen subcarriers.

Bits to symbol component 430 may map the bits to symbols on the complexplane. For example, bits to symbol component 430 may map four bits to asymbol in the dual-polarization QPSK constellation. Overlap and savebuffer 440 may buffer 256 symbols. Overlap and save buffer 440 mayreceive 128 symbols at a time from bits to symbol component 430. Thus,overlap and save buffer 440 may combine 128 new symbols, from bits tosymbol component 430, with the previous 128 symbols received from bitsto symbol component 430.

FFT component 450 may receive 256 symbols from overlap and save buffer440 and convert the symbols to the frequency domain using, for example,a fast

Fourier transform (FFT). FFT component 450 may form 256 frequency binsas a result of performing the FFT. Replicator component 460 mayreplicate the 256 frequency bins or registers to form 512 frequency bins(e.g., for T/2 based filtering of the subcarrier). This replication mayincrease the sample rate.

Pulse shape filter 470 may apply a pulse shaping filter to the datastored in the 512 frequency bins to thereby provide the digitalsubcarriers which are multiplexed and subject to an inverse FFT, asdescribed below. The purpose of pulse shape filter 470 is to calculatethe transitions between the symbols and the desired spectrum so that thesubcarriers can be packed together on the channel. Pulse shape filter470 may also be used to introduce timing skew between the subcarriers tocorrect for timing skew induced by links between nodes 10, 20, 30, and40 in network 100. Mux component 480 may receive all four, eight Gbaudsubcarriers (from the four pulse shape filters 470) and multiplex themtogether to form a 2048 element vector.

IFFT component 490 may receive the 2048 element vector and return thesignal back to the time domain, which may now be at 64 GSample/s. IFFTcomponent 490 may convert the signal to the time domain using, forexample, an inverse fast Fourier transform (IFFT). Take last 1024component 495 may select the last 1024 samples from IFFT component 490and output the 1024 samples to DACs 320 at 64 GSample/s, for example.

While FIG. 4 shows TX DSP 310 as including a particular quantity andarrangement of functional components, in some implementations, TX DSP310 may include additional functional components, fewer functionalcomponents, different functional components, or differently arrangedfunctional components.

FIG. 5 illustrates an example of subcarriers SC1 to SC4 that may beoutput from transmitter 11 (similar subcarriers may be output fromtransmitters in transceivers located at other nodes). As shown in FIG.5, subcarriers SC1 to SC4 may not spectrally overlap with one anotherand may be, for example, Nyquist subcarriers, which may have a frequencyspacing equal to or slightly larger than the individual subcarrierbaud-rate.

As further shown in FIG. 5, subcarriers may have spectra or bandwidths,S3 (subcarrier SC3) and S4 (subcarrier SC4) above frequency f0, whichmay correspond to a center frequency of the laser (e.g., laser 508). Inaddition, subcarriers may have spectra or bandwidths, S1 (subcarrierSC1) and S2 (subcarrier SC2) below frequency f0. In one example, thenumber of subcarriers equals a number of the independent input datastreams, such as, in the present example (Client Data 1 to 4—fourindependent input data streams and SC1 to SC4—four subcarriers).

As noted above, in one example, subcarriers output from transmitter 301may be supplied to splitter 12, which may have ports that supply a powersplit portion of each subcarrier to a corresponding combiner input portat another node, such as combiner input port 24-1 at node 20, combinerinput port 34-1 of node 30, and combiner input port 44-1 of node 40.Each combiner (14, 24, 34, and 44) at each node combines power splitsubcarrier portions from selected splitter output ports of other nodes.The combiner at each node combines such power split subcarrier portionsand supplies the combined subcarriers to a receiver, such as receiver302, which may select data carried by one of such subcarriers, asdescribed in greater detail below with reference to FIGS. 6 and 7.

As shown in FIG. 6, optical receiver 301 may include a polarizationsplitter 605 (having first (605-1) and second (605-2) outputs), a localoscillator laser 610, 90 degree optical hybrids or mixers 620-1 and620-2 (referred to generally as hybrid mixers 620 and individually ashybrid mixer 620), detectors 630-1 and 630-2 (referred to generally asdetectors 630 and individually as detector 630, each including either asingle photodiode or balanced photodiode), AC coupling capacitors 632-1and 632-2, transimpedance amplifiers/automatic gain control circuitsTIA/AGC 634-1 and 634-2, ADCs 640-1 and 640-2 (referred to generally asADCs 640 and individually as ADC 640), and an RX DSP 650. Localoscillator 610, hybrid mixers 620, detectors 630, ADCs 640, and RX DSP650 may correspond to like components described with regard to FIG. 6.

Polarization beam splitter (PBS) 605 may include a polarization splitterthat splits an input signal into two orthogonal polarizations, such asthe first polarization and the second polarization. Hybrid mixers 620may combine the polarization signals with light from local oscillatorlaser 610. For example, hybrid mixer 620-1 may combine a firstpolarization signal (e.g., the component of the incoming optical signalhaving a first or TE polarization output from PBS port 605-1) with theoptical signal from local oscillator 610, and hybrid mixer 620-2 maycombine a second polarization signal (e.g., the component of theincoming optical signal having a second or TM polarization output fromPBS port 605-2) with the optical signal from local oscillator 610. Inone example, a polarization rotator may be provided at PBS output 605-2to rotate the second polarization to be the first polarization.

Detectors 630 may detect mixing products output from the opticalhybrids, to form corresponding voltage signals, which are subject to ACcoupling by capacitors 632-1 and 632-1, as well as amplification andgain control by TIA/AGCs 634-1 and 634-2. The outputs of TIA/AGCs 634-1and 634-2 and ADCs 640 may convert the voltage signals to digitalsamples. For example, two detectors or photodiodes 630-1 may detect thefirst polarization signals to form the corresponding voltage signals,and a corresponding two ADCs 640-1 may convert the voltage signals todigital samples for the first polarization signals after amplification,gain control and AC coupling. Similarly, two detectors 630-2 may detectthe second polarization signals to form the corresponding voltagesignals, and a corresponding two ADCs 640-2 may convert the voltagesignals to digital samples for the second polarization signals afteramplification, gain control and AC coupling. RX DSP 650 may process thedigital samples for the first and second polarization signals togenerate resultant data, which may be outputted as output data.

While FIG. 6B shows optical receiver 302 as including a particularquantity and arrangement of components, in some implementations, opticalreceiver 302 may include additional components, fewer components,different components, or differently arranged components. The quantityof detectors 630 and/or ADCs 640 may be selected to implement an opticalreceiver 302 that is capable of receiving a polarization diverse signal.In some instances, one of the components illustrated in FIG. 6 mayperform a function described herein as being performed by another one ofthe components illustrated in FIG. 6.

Consistent with the present disclosure, in order to select a particularsubcarrier at a remote node, local oscillator 610 may be tuned to outputlight having a wavelength relatively close to the selected subcarrierwavelength to thereby cause a beating between the local oscillator lightand the selected subcarrier. Such beating will either not occur or willbe significantly attenuated for the other non-selected subcarriers sothat data carried by the selected subcarrier is detect and processed byDSP 650. In the example shown in FIG. 6, appropriate tuning of the localoscillator wavelength enables selection of one of the subcarriers, e.g.,SC1, carrying signals or data indicative of Client Data 1. Accordingly,subcarriers may be effectively routed through network 100 to a desiredreceiver in a particular node.

Accordingly, at each node receiver, such as receiver 302, which may beincluded in nodes 10, 20, 30, and 40, the local oscillator laser, e.g.,610, may be tuned to have a wavelength close to that of one of thesubcarrier carrying signals and data indicative of the desired clientdata to be output from the DSP, e.g., DSP 650. Such tuning may beachieved by adjusting a temperature or current flowing through localoscillator 610, which may include a semiconductor laser, such as adistributed feedback (DFB) laser or distributed Bragg reflector (DBR)laser. Thus, different optical components in each receiver to selectoptical signals carrying a desired data stream are not required. Rather,as noted above, the same or substantially the same circuitry may beproved in the receiver portion of each node, such as a node in a meshnetwork, and signal or data selection may be achieved by tuning thelocal oscillator laser to the desired beating wavelength.

As further shown in FIG. 6, DSP 650 may have an output 652, such thatbased on such output, the temperature of or the current supplied tolocal oscillator laser 610 may be controlled. In the case of temperaturecontrol, a thin film heater may be provided adjacent local oscillatorlaser 610, and an appropriate current may be supplied to such heater,based on output 652, to heat laser 610 to the desired temperature.Control circuitry in DSP 650 may generate output or control signal 652.Alternatively, such circuitry may be provided outside DSP 650.

A receiver 700 consistent with an additional aspect of the presentdisclosure will next be described with reference to FIG. 7. Receiver 700may include an optical filter 702 configured to receive optical signalsfrom either PBS output 702-1 or 702-2. In one example, optical filter702 may be configured to pass the desired subcarrier, e.g., SC1, whileblocking the remaining subcarriers transmitted with SC1, as well asother subcarriers other subcarriers transmitted from other nodes.Optical filter 702 may include, for example, a wavelength selectiveswitch (WSS), arrayed waveguide grating (AWG), Bragg grating, and/or adichroic filter.

As further shown in FIG. 7, the selected optical subcarrier may besupplied to a PIN photodiode 704, for example, which converts thereceived optical subcarrier to an electrical signal. The electricalsignal may then be subject to AC coupling by capacitor 706 andamplification and automatic gain control by TIA/AGC 708, followed byconversion to digital samples in 710 and digital signal processing byDSP 714 to output the data carried by the selected subcarrier, e.g.,Client Data 3. In the example shown in FIG. 7, however, DSP 714 isoptional. DSP 714 may include circuitry for carrying out feedforwardequalization (FFE), decision feedback equalization (DFE) or maximumlikelihood sequence estimate (MLSE) equalization.

In the example discussed above in connection with FIG. 6, the opticalsignals are subject to coherent detection of QPSK or QAM modulatedoptical signals. Consistent with a further aspect of the presentdisclosure, DSP 310 appropriately configured in conjunction with DACs320 may operate to supply drive signals to modulators 340, such that themodulator outputs intensity modulated optical subcarriers, such as .Such subcarriers are then sensed with the direct detection, as shown inFIG. 7 whereby the optical subcarrier is supplied to a photodiode, asopposed to be mixing with local oscillator light, as in the coherentdetection scheme shown in FIG.

6.

Consistent with an additional aspect of the present disclosure, based onan appropriate configuration of DSP 310 in conjunction with the laser,modulators and other circuitry discussed above, certain subcarriers(e.g., SC1 and SC3) may be intensity modulated, a pulse amplitudemodulation (PAM, such as PAM-4), for example while others (SC2 and SC4,for example) may be modulated in accordance with binary phase shiftkeying (BPSK), QPSK, or m-QAM modulation formats, where m is an integer.In addition, certain subcarriers (e.g., SC1 and SC3) may be modulated inaccordance with a first QAM modulation format, such as one of BPSK,QPSK, 16-QAM, 64-QAM, 256-QAM or some other m-QAM modulation formatwhile other subcarriers (e.g., SC2 and SC4) may be modulated inaccordance with a second modulation format. Moreover, each subcarriermay be modulated to carry data at different rates, e.g., subcarrier SC1may carry data at a first rate and subcarrier SC2 may carry data at adifferent rate that is higher or lower than the first rate. In addition,each subcarrier may be modulated to carry data with different baudrates, e.g., subcarrier SC1 may carry data at or have an associated afirst baud rate and subcarrier SC2 may carry data at or have anassociated second baud rate that is higher or lower (different) than thefirst baud rate.

Accordingly, as noted above, a simplified and less expensive transmittermay be realized consistent with the present disclosure in which a laserand modulator may be employed to generate multiple subcarriers, wherebyeach of which may be detected and the client data associated therewithmay be output from receivers provided at different locations in a meshnetwork, for example. Improved network flexibility can therefor beachieved.

Other embodiments will be apparent to those skilled in the art fromconsideration of the specification. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

1-24 (canceled)
 25. A system, comprising: a transmitter including: aplurality of forward error correction encoders, each of which beingoperable to receive a respective one of a plurality of independent datastreams and being operable to provide a corresponding one of a pluralityof encoded signals, and a modulator, including a Mach-Zehnder modulator,operable to modulate light based on the plurality of encoded signals tosupply a modulated optical including a plurality of Nyquist subcarriers,a first one of the plurality of Nyquist subcarriers carries dataindicative of a first one of the plurality of independent data streams,and a second one of the plurality of Nyquist subcarriers carries dataindicative of a second one of the plurality of independent data streams;a first receiver operable to receive a first portion of a first one ofthe plurality of Nyquist subcarriers and a first portion of the secondone of the plurality of Nyquist subcarriers, the first receiver beingoperable to process the first portion of the first one of the pluralityof Nyquist subcarriers and being operable to output the first one theplurality of independent data streams; and a second receiver operable toreceive a second portion of the first one of the plurality of Nyquistsubcarriers and a second portion of the second one of the plurality ofNyquist subcarriers, the second receiver being operable to process thesecond portion of the second one of the plurality of Nyquist subcarriersand being operable to output the second one the plurality of independentdata streams.